Neuroscience Letters 573 (2014) 19–23

Contents lists available at ScienceDirect

Neuroscience Lettersjournal homepage: www.elsevier.com/locate/neulet

Intrastriatal grafts of fetal ventral mesencephalon improve
allodynia-like withdrawal response to mechanical stimulation
in a rat model of Parkinson’s disease
Ryuichiro Takeda a,b , Yasushi Ishida b,∗ , Kosuke Ebihara b , Hiroshi Abe b , Hisae Matsuo b ,Tetsuya Ikeda c , Go Koganemaru b , Aki Kuramashi b , Hideki Funahashi b , Yasuhiro Magata d ,Keiichi Kawai e , Toshikazu Nishimori baHealthcare and Security center, University of Miyazaki, 1-1 Gakuen Kibanadai-nishi, Miyazaki 889-1692, Japan
Department of Psychiatry, Faculty of Medicine, University of Miyazaki, 5200 Kihara, Kiyotake, Miyazaki 889-1692, Japanc
Division of Neurobiology, Faculty of Medicine, University of Miyazaki, 5200 Kihara, Kiyotake, Miyazaki 889-1692, JapandLaboratory of Genome Bio-Photonics, Photon Medical Research Center, Hamamatsu University School of Medicine, 1-20-1 Handayama,
Hamamatsu 431-3192, Japane
Department of Quantum Medical Technology, Division of Health Sciences, Graduate School of Medical Sciences, Kanazawa University, 5-11-80 Kodatsuno,
Kanazawa 920-0942, Japanbh i g h l i g h t s• A 6-OHDA rat model of Parkinson’s disease showed allodynia-like withdrawal response.
• Fetal ventral mesencephalon (VM) cells were transplanted into the denervated striatum.
• The withdrawal threshold was increased in 6-OHDA rats with VM grafts.
• Transplantation of VM cells may ameliorate pain sensation in 6-OHDA rats.

a r t i c l e

i n f oArticle history:
Received 17 March 2014
Received in revised form 2 May 2014
Accepted 3 May 2014
Available online 14 May 2014
Keywords:
Pain
6-Hydroxydopamine
Basal ganglia
Transplantation
Immunohistochemistrya b s t r a c t
We previously reported that a unilateral 6-hydroxydopamine (6-OHDA) rat model of Parkinson’s disease
showed allodynia-like withdrawal response to mechanical stimulation of the ipsilateral side of the rat
hindpaw. The goal of this study was to investigate the effect of intrastriatal grafts of fetal ventral mesencephalon (VM) on the withdrawal response in 6-OHDA rats. The withdrawal threshold in response to
the mechanical stimulation of the rat hindpaw was measured using von Frey ﬁlaments. In the ipsilateralside of the 6-OHDA lesions, the withdrawal threshold in response to mechanical stimulation signiﬁcantly
increased in 6-OHDA rats with VM grafts compared with those with sham grafts, but did not change inthe contralateral side at 5 weeks after transplantation. The present results suggest that the intrastriatal
grafts of fetal VM may relieve pain sensation induced by mechanical stimulation in 6-OHDA rats.

1. Introduction
Parkinson’s disease (PD) is well characterized by dominating
bradykinesia, rigidity, tremors, postural instability, and dementia
as clinical features [20]. Apart from parkinsonian motor symptoms, PD is less widely considered a disease that causes a large
variety of pain syndromes with a prevalence of ∼40% [7]. Another∗ Corresponding author. Tel.: +81 985852969; fax: +81 985855475.
E-mail address: ishiday@med.miyazaki-u.ac.jp (Y. Ishida).
http://dx.doi.org/10.1016/j.neulet.2014.05.007
0304-3940/© 2014 Elsevier Ireland Ltd. All rights reserved.

© 2014 Elsevier Ireland Ltd. All rights reserved.

recent clinical examination involving 176 home-living PD patients
showed that 146 (83%) of the patients had pain-related symptoms[3]. However, little is known about the clinical predictors of pain
in PD, and it remains unclear whether these painful symptoms are
inherent in this disease or are caused by medication [24].
Rats with unilateral 6-hydoroxydopamine (6-OHDA) lesionsof the mesostriatal dopamine pathway have been widely used
as a rodent model for PD [26]. The 6-OHDA rat is valuable forclarifying the mechanisms of painful symptoms in patients with
PD. The response of pain-related behaviors in this model animal is different from that in a naïve control animal. Unilateral

R. Takeda et al. / Neuroscience Letters 573 (2014) 19–23

injections of 6-OHDA into the rat substantia nigra (SN) decreased
the ﬂinch-jump threshold of electrical footshock [5] and the latencyof withdrawal responses in hot plate, paw pressure, and paw
immersion tests [23]. We previously reported that a unilateral 6OHDA-lesioned rat showed allodynia-like withdrawal response tomechanical stimulation of the ipsilateral side of the rat hindpaw[29]. These previous reports suggested that unilateral dopamine
depletion might cause hypersensitivity to mechanical or noxious
stimulus.
Many reports have been made in which intrastriatal grafts offetal ventral mesencephalon (VM) could ameliorate motor function
and neurochemical milieu in the brain of unilateral 6-OHDAlesioned rats [12,13,16,20,21]; however, there is no evidence thatneural transplantation affects pain-related behavior in PD model
animals. Thus, the purpose of this study was to evaluate whether
the intrastriatal transplants of fetal VM affect the allodynialike withdrawal response to mechanical stimulation in 6-OHDA
rats.

methanphetamine-induced
rotaion (turns/min)20

lesion plus sham graft (n=8)
lesion plus VM graft (n=7)
10
8
6
4

**

2
transplantation

0
0

-2

4

weeks post-transplantation
Fig. 1. Mean numbers of rotations (±SEM) ipsilateral to 6-OHDA lesions per min
over 60 min test period in response to methamphetamine (3 mg/kg, i.p.) are shownfor lesion plus sham graft group and lesion plus VM graft group. Ipsilateral rotation
found in lesion plus VM graft group signiﬁcantly decreased at 4 weeks after transplantation as compared to lesion plus sham graft group (**P < 0.01, two way ANOVA
with repeated measures followed by Bonferroni post hoc tests).

2. Materials and methods
ﬁve applications, were deﬁned as the threshold against mechanicalstimulation.
After the behavioral test with mechanical stimulation, theseanimals were deeply anesthetized with an overdose of sodium pentobarbital (i.p.) and intracardially perfused with saline followed by
4% paraformaldehyde in a 0.1 M phosphate buffer (PB) for 30 min.

normal (n=6)
lesion plus sham graft (n=8)
lesion plus VM graft (n=7)

withdrawal threshold (g)

A
80
60

#

40
20

0

**
**
-1

*

transplantation

0

5

B
withdrawal threshold (g)Male Wistar rats (Charles River, Japan), weighting 120–130 g
at the beginning of this study, were used. They were housed ina group of three or four per cage under a 12 h light/dark cyclewith free access to food and water. The experimental proceduresin this study were performed in strict adherence of the Guidelinesof the University of Miyazaki for the care and use of experimental animals and under the approval of the ethical committee ofanimal experimentation at the University of Miyazaki. A unilateral lesion of the left medial forebrain bundle (MFB) was made bymicroinjection of 10 ␮g 6-OHDA hydrobromide (Sigma, MA, USA)in 5 ␮l of sterile saline containing 0.01% ascorbic acid per animal,
as previously described [12,13,15]. Stereotaxic coordinates for thelesions were 3.3 mm rostral to the interaural line, 1.3 mm left ofthe midline, and 6.8 and 6.4 mm (injected 2.5 ␮l each) ventral tothe dural surface [17]. Motor disturbance was assessed by counting
full-rotations per min in a cylindrical container (30 cm diameter)at 10-min intervals for the ﬁrst 60 min after methamphetamine(3 mg/kg, i.p.) administration [21]. Behavioral screening was carried out after 2 weeks of recovery, and animals that turned noless than 5 turns/min on the methamphetamine challenge wereused as the 6-OHDA rats in the study. There was an immediate
and almost complete destruction of dopamine neurons of the SN
and of the ventral tegmental area (VTA), resulting in the near total
depletion (2% of normal) of dopamine in the striatum ipsilateral
to the 6-OHDA injections [12]. Seven and eight animals out of the
6-OHDA rats were allocated as the lesion plus VM graft group andlesion plus sham graft group, respectively. Neural transplantation
was performed from fetal VM tissue according to the cell suspension method [4,8]. Six ␮l of the suspension containing 9.6 × 105
cells from 15-day-old rat embryos was injected into two sites (3 ␮leach site) of the denervated striatum: (1) AP = 1.0 mm rostral tothe bregma, L = 2.5 mm left of midline, V = 5.0 mm ventral to thedural surface and (2) AP = 0.0 mm to the bregma, L = 3.2 mm left of
midline, V = 5.0 mm ventral to the dural surface [22]. For animals
receiving sham grafts, 3 ␮l aliquots of saline alone were injected
at the same sites. All of the VM and sham grafted rats were tested
again for rotational asymmetry at 4 weeks after transplantation.
One week before and 5 weeks after transplantation, the withdrawal threshold in response to mechanical stimulation of the rat
hindpaw was measured using von Frey ﬁlaments in normal (n = 6),
lesion plus sham grafted (n = 8), and lesion plus VM grafted animals(n = 7). The maximum and minimum cut-offs of the withdrawal
thresholds were set at 100 and 1 g, respectively. The withdrawal
responses using the von Frey ﬁlaments, at least two times out of

80
60
40
20
transplantation

0
-1

5
0
weeks post-transplantation

Fig. 2. Withdrawal threshold in response to mechanical stimulation of rat hindpaw by using von Frey ﬁlaments, (A) ipsilateral and (B) contralateral to 6-OHDA
lesions and transplantation. Withdrawal threshold was examined in hindpaw bilaterally 1 week before and 5 weeks after transplantation for normal control rats,
lesion plus sham grafted rats, and lesion plus VM grafted rats. Results are shownas means ± SEM; *P < 0.05, **P < 0.01 compared to each corresponding value in normal group (two way ANOVA with repeated measures followed by Bonferroni post
hoc tests); #P < 0.05 compared to each corresponding value of lesion plus sham graftgroup (two way ANOVA with repeated measures followed by Bonferroni post hoc
tests).

R. Takeda et al. / Neuroscience Letters 573 (2014) 19–23

The brain was removed, postﬁxed for 1 h in the same ﬁxativeand cryoprotected in 10% sucrose for 1 h and 30% sucrose in PB
overnight. Coronal serial sections at the SN and striatum, 50 ␮mthick, were frozen, cut, and collected in phosphate-buffered saline(pH 7.4) and processed for immunohistochemical staining for tyrosine hydroxylase (TH) as free-ﬂoating sections [15,16,29].
The data were analyzed using two way analysis of variance(ANOVA) with repeated measures (groups × time) followed by aBonferroni post hoc test. P values <0.05 were regarded as being
statistically signiﬁcant.
3. Results
Before transplantation, methamphetamine (3 mg/kg, i.p.)
induced strong ipsilateral rotation in the 6-OHDA-lesioned rats. In
the lesion plus sham grafted animals (n = 8), methamphetamineinduced rotation continued to be observed 8 weeks following
lesion generation. This behavior was signiﬁcantly suppressed
at 4 weeks after transplantation of the intrastriatal VM grafts(n = 7) [main effect of groups, F1,13 = 2.09, P = 0.17; main effect oftime, F1,13 = 29.38, P < 0.01; groups × time interaction, F1,13 = 13.87,
P < 0.01] (Fig. 1).
Before transplantation, the 6-OHDA rats (in both lesion plus
sham graft and lesion plus VM graft groups) showed signiﬁcantly
lower withdrawal thresholds in response to mechanical stimulation of the hindpaw ipsilateral to the lesion compared with thenormal control rats (Fig. 2A) but did not change in the contralateral side (Fig. 2B). In the ipsilateral side of the 6-OHDA lesions,
the withdrawal thresholds in response to mechanical stimulation
was signiﬁcantly increased in the 6-OHDA rats with VM grafts

21

compared with those with sham grafts [main effect of groups,
F2,18 = 22.92, P < 0.01; main effect of time, F1,18 = 4.87, P = 0.04;
groups × time interaction, F2,18 = 3.26, P = 0.06] (Fig. 2A) but did notchange in the contralateral side at 5 weeks after transplantation[main effect of groups, F2,18 = 3.38, P = 0.06; main effect of time,
F1,18 = 2.10, P = 0.16; groups × time interaction, F2,18 = 0.32, P = 0.72](Fig. 2B).
Consistent with previous studies [15,16], injection of 6-OHDA
into the left MFB produced a marked reduction in TH immunoreactivity in the left SN and VTA, while there was abundant TH
immunoreactivity in these two regions contralateral to the lesion(Fig. 3B). Similarly, TH immunoreactivity in the left striatum wasremarkably reduced, whereas immunoreactivity in the contralateral side was localized in abundance for each 6-OHDA rat in thesham graft group (Fig. 3A). The VM grafts were all located almost in
the middle part of the striatum lateromedially and surrounded bya rich TH-positive ﬁber network. However, it appeared that graftderived ﬁber outgrowth was restricted to the area close to the graft(Fig. 3C and D).
4. Discussion
The present results of the von Frey test seem to be similar to
those from our previous study in which a 6-OHDA rat elicited the
reduction in withdrawal latency in response to mechanical stimulation of its hindpaw [29]. As we found for the ﬁrst time in thepresent study, the allodynia-like withdrawal response to mechanical stimulation signiﬁcantly improved in the rat hindpaw on theipsilateral side 5 weeks after transplantation in the lesion plus VM
graft group, whereas there was no signiﬁcant change in the lesion

Fig. 3. Immunohistochemical visualization of tyrosine hydroxylase (TH) of rats with unilateral 6-OHDA lesions plus sham grafts (A and B) or VM grafts (C and D). (A)striatal region of sham grafted rat; (B) mesencephalon of sham grafted rat; (C) striatal region of VM grafted rat; (D) TH positive cells at higher magniﬁcation in VM graft.
Lesioned/grafted side is on left in each section of (A–C). Boxed area in panel C is shown at high magniﬁcation in panel D. Scale bars = 1 mm (A–C), 100 ␮m (D).

22

R. Takeda et al. / Neuroscience Letters 573 (2014) 19–23

plus sham graft group. These ﬁndings suggest that the intrastriatal
transplants of fetal VM in unilateral 6-OHDA-lesioned rats could
ameliorate not only motor but also sensory dysfunction in nociceptive processing.
As reviewed by Hagelberg et al. [10], striatal dopamine D2 receptors are involved in the regulation of pain in humans. Positronemission tomography indicated that low D2 receptor availability
in the striatum of healthy subjects (indicating either a low density of D2 receptors or a high synaptic concentration of dopamine)is associated with a high cold pain threshold and a low capacity
to recruit central pain inhibition by conditioned stimulation [10].
They also indicated that patients with chronic orofacial pain havehigher D2 receptor availability than their age-matched controls.
Unilateral 6-OHDA lesions of the mesostriatal dopamine pathwaycause striatal upregulation of D2 receptors in rats, which is evident
even at 3 days after 6-OHDA lesions [14]. The upregulation of D2
receptors caused by 6-OHDA lesions can be reversed by intrastriatal dopamine-rich grafts of fetal VM [25]. In VM grafted striatum,
the extracellular concentration of dopamine was restored to 67%
from 2% of normal in our previous study [12], which was carried out with similar methods as those in the present study. The
restoration of dopamine levels might contribute to the reversal of
D2 receptor upregulation, at least in part, in VM grafted striatum.
Taken together, the present results are consistent with the previous ﬁndings, in which dopamine depletion and its restoration in themesostriatal system affect the withdrawal response to mechanicalstimulus.
However, the mechanisms of allodynia-like behavior on theside ipsilateral to the 6-OHDA lesions remain unclear. We are currently focusing on another possibility, unilateral impairment of
the descending cathecholaminergic system involved in the A11
area. Because A11 descending ﬁbers predominantly uncross [11,27]
and the A11 cell group may provide a major source of dopaminergic innervation of the spinal cord that affects somatosensory
processing [9], it can be speculated that the unilateral 6-OHDA
lesions affect the impairment of the descending inhibition system
involved in the A11 area, eliciting allodynia-like behavior on theside ipsilateral to the 6-OHDA lesions.
Both dopaminergic and nondopaminergic neurotransmissions
may be involved in these mechanisms. The basal ganglia, speciﬁcally the striatum, are among the neuronal structures with highest
concentrations of endogenous opiates and their receptors, which
may be relevant to the mechanisms of endogenous analgesia [2].
The 6-OHDA lesioning decreases striatal opioid (␮, ␦, ␬) binding site
density, increases proenkephalin mRNA, and decreases prodynorphin mRNA [28]. Tissue levels of substance P and neurokinin A havebeen reported to decrease in the SN and/or striatum in 6-OHDA
rats, and simultaneous increases in both tissue and extracellular concentrations of GABA have been reported in the denervatedstriatum [18]. Therefore, alteration in these peptides (enkephalin,
dynorphin, substance P, neurokinin A) and GABA activities in themesostriatal system may have some effect on the alteration in thewithdrawal response to mechanical stimulation in 6-OHDA rats.
Transplantation of fetal nigral cells was reported to reverse the
6-OHDA lesion-induced alteration of preproenkephalin, prodynorphin, and preprotachykinin mRNA levels [1,6] and Met-enkephalin
levels [19]. From the present study, the nondopaminergic neuromodulation due to intrastriatal transplantation might be related to
the reversal of the allodynia-like withdrawal response in 6-OHDA
rats.
Although further studies are needed to elucidate the role ofthe dopaminergic and nondopaminergic systems involved in sensory and/or motor processing in mesostriatum, we demonstrated
that intrastriatal dopaminergic transplants might have, at least inpart, a therapeutic potential to mitigate pain-related symptoms in
PD.

References[1] A. Bal, M. Savasta, M. Chritin, F. Mennicken, D.N. Abrous, M. Le Moal, C. Feuerstein, J.P. Herman, Transplantation of fetal nigral cells reverses the increase of
preproenkephalin mRNA levels in the rat striatum caused by 6-OHDA lesionof the dopaminergic nigrostriatal pathway: a quantitative in situ hybridization
study, Mol. Brain Res. 18 (1993) 221–227.
[2] A.C. Barceló, B. Filippini, J.H. Pazo, The striatum and pain modulation, Cell Mol.
Neurobiol. 32 (2012) 1–12.
[3] A.G. Beiske, J.H. Loge, A. Rønningen, E. Svensson, Pain in Parkinson’s disease:prevalence and characteristics, Pain 141 (2009) 173–177.
[4] A. Björklund, S.B. Dunnett, Neural transplantation in adult rats, in: S.B. Dunnett, A. Björklund (Eds.), Neural Transplantation. A Practical Approach, Oxford
University Press, New York, 1992, pp. 57–78.
[5] R.J. Carey, Acute ipsilateral hyperalgesia and chronic contralateral hypoalgesia
after unilateral 6-hydroxydopamine lesions of the substantia nigra, Exp. Neurol.
91 (1986) 277–284.
[6] M.A. Cenci, K. Campbell, A. Björklund, Neuropeptide messenger RNA expression
in the 6-hydroxydopamine-lesioned rat striatum reinnervated by fetal dopaminergic transplants: differential effects of the grafts on preproenkephalin,preprotachykinin and prodynorphin messenger RNA levels, Neuroscience 57(1993) 275–296.
[7] K.R. Chaudhuri, A.H. Schapira, Non-motor symptoms of Parkinson’s disease: dopaminergic pathophysiology and treatment, Lancet Neurol. 8 (2009)464–474.
[8] S.B. Dunnett, A. Björklund, Staging and dissection of rat embryos, in: S.B. Dunnett, A. Björklund (Eds.), Neural Transplantation. A Practical Approach, Oxford
University Press, New York, 1992, pp. 1–19.
[9] S.M. Fleetwood-Walker, P.J. Hope, R. Mitchell, Antinociceptive actions ofdescending dopaminergic tracts on cat and rat dorsal horn somatosensory
neurons, J. Physiol. 399 (1988) 335–348.
[10] N. Hagelberg, S.K. Jääskeläinen, I.K. Martikainen, H. Mansikka, H. Forssell, H.
Scheinin, J. Hietala, A. Pertovaara, Striatal dopamine D2 receptors in modulationof pain in humans: a review, Eur. J. Pharmacol. 500 (2004) 187–192.
[11] T. Hökfelt, O. Phillipson, M. Goldstein, Evidence for a dopaminergic pathway
in the rat descending from the A11 cell group to the spinal cord, Acta Physiol.
Scand. 107 (1979) 393–395.
[12] Y. Ishida, H. Hashiguchi, K. Todaka, I. Kuwahara, Y. Ishizuka, H. Nakane, D.
Uchimura, T. Nishimori, Y. Mitsuyama, Serotonergic activity in the rat striatum
after intrastriatal transplantation of fetal nigra as measured by microdialysis,
Brain Res. 788 (1998) 207–214.
[13] Y. Ishida, K. Kawai, Y. Magata, K. Ebihara, R. Takeda, H. Abe, M. Yoshimoto, H.
Hashiguchi, K. Odagiri, H. Matsuo, T. Nishimori, Differential expression of Fosand Zif268 in the nigrostriatal system after methamphetamine administration
in a rat model of Parkinson’s disease, Synapse 62 (2008) 920–926.
[14] Y. Ishida, K. Kawai, Y. Magata, H. Abe, M. Yoshimoto, R. Takeda, H. Hashiguchi,
T. Mukai, H. Saji, Alteration of striatal [11 C]raclopride and 6-[18 F]ﬂuoro-l-3,4dihydroxyphenylalanine uptake precedes development of methamphetamineinduced rotation following unilateral 6-hydroxydopamine lesions of medial
forebrain bundle in rats, Neurosci. Lett. 389 (2005) 30–34.
[15] Y. Ishida, I. Kuwahara, K. Todaka, H. Hashiguchi, T. Nishimori, Y. Mitsuyama,
Dopaminergic transplants suppress l-DOPA-induced Fos expression in the
dopamine-depleted striatum in a rat model of Parkinson’s disease, Brain Res.
727 (1996) 205–211.
[16] Y. Ishida, K. Todaka, H. Hashiguchi, R. Takeda, Y. Mitsuyama, T. Nishimori, Morphological changes in immunopositive cells of ionotropic glutamate receptor
subunits during the development of transplanted fetal ventral mesencephalicneurons, Brain Res. 940 (2002) 79–85.
[17] J.F.R. König, R.A. Klippel, The Rat Brain. A Stereotaxic Atlas of the Forebrain andLower Parts of the Brain Stem, Williams and Wilkins, Baltimore, 1963.
[18] N. Lindefors, E. Brodin, U. Tossman, J. Segovia, U. Ungerstedt, Tissue levels andin vivo release of tachykinins and GABA in striatum and substantia nigra of rat
brain after unilateral striatal dopamine denervation, Exp. Brain Res. 74 (1989)
527–534.
[19] M. Manier, D.N. Abrous, C. Feuerstein, M. Le Moal, J.P. Herman, Increase of
striatal methionin enkephalin content following lesion of the nigrostriatal
dopaminergic pathway in adult rats and reversal following the implantation ofembryonic dopaminergic neurons: a quantitative immunohistochemical analysis, Neuroscience 42 (1991) 427–439.
[20] J. Mokry, Experimental models and behavioural tests used in the study ofParkinson’s disease, Physiol. Res. 44 (1995) 143–150.
[21] H. Nishino, T. Hashitani, M. Kumazaki, H. Sato, F. Furuyama, Y. Isobe, N. Watari,
M. Kanai, S. Shiosaka, Long-term survival of grafted cells, dopamine synthesis/release, synaptic connections, and functional recovery after transplantationof fetal nigral cells in rats with unilateral 6-OHDA lesions in the nigrostriatal
dopamine pathway, Brain Res. 534 (1990) 83–93.
[22] G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press,
San Diego, 1997.
[23] N.F. Saadé, S.F. Atweh, N.B. Bahuth, S.J. Jabbur, Augmentation of nociceptive
lesions of either dopaminergic terminals or midbrain dopaminergic neurons,
Brain Res. 751 (1997) 1–12.
[24] R. Sandyk, C.R. Bamford, R.P. Iacono, Pain and sensory symptoms in Parkinson’s
disease, Int. J. Neurosci. 39 (1988) 15–25.
[25] M. Savasta, F. Mennicken, M. Chritin, D.N. Abrous, C. Feuerstein, M. Le Moal, J.P.
Herman, Intrastriatal dopamine-rich implants reverse the changes in dopamine

R. Takeda et al. / Neuroscience Letters 573 (2014) 19–23
D2 receptor densities caused by 6-hydroxydopamine lesion of the nigrostriatal
pathway in rats: an autoradiographic study, Neuroscience 46 (1992) 729–738.
[26] R.K.W. Schwarting, J.P. Huston, The unilateral 6-hydroxydopamine lesion
model in behavioral brain research. Analysis of functional deﬁcits, recovery
and treatments, Prog. Neurobiol. 50 (1996) 275–331.
[27] G. Skagerberg, A. Björklund, O. Lindvall, R.H. Schmidt, Origin and terminationof the diencephalo-spinal dopamine system in the rat, Brain Res. Bull. 9 (1982)
237–244.

23[28] J.A. Smith, F.M. Leslie, R.S. Broide, S.E. Loughlin, Long-term changes in striatal opioid systems after 6-hydroxydopamine lesion of rat substantia nigra,
Neuroscience 55 (1993) 935–951.
[29] R. Takeda, T. Ikeda, F. Tsuda, H. Abe, H. Hashiguchi, Y. Ishida, T. Nishimori, Unilateral lesions of mesostriatal dopaminergic pathway alters the withdrawal
response of the rat hindpaw to mechanical stimulation, Neurosci. Res. 52 (2005)
31–36.